still failed . Is that "install with -Syu " not means "upgrade the whole system "? I just wanna install single package .
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holysDec 1 '12 at 3:55

@cdh try the dirty hack then, and remove that afterwards; and -Syu means upgrade whole system yes, but in this case, it means upgrade whole system while install the desired package
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warl0ckDec 1 '12 at 3:57

The problem seems to be that pacman isn't checking the right key... Indeed, the libproxy package is not signed by Evangelos Foutras, but by Jan de Groot. Maybe E.F. was the previous maintainer, and your pacman hasn't acknowledged the change.

You might want to download a fresh copy of the master package list first: